Improve DX of template_preprocess_pager

Problem/Motivation

The body of the function template_preprocess_pager is 128 lines long and is hard to parse or manipulate.

It seems to be broken up in to a number of logical parts, some of which even have comments to indicate where the current step ends.

1. Initialize local variables
2. Early returns
3. Calculate various markers
4. Prepare for generation loop
5. Create the "first" and "previous" links
6. Add an ellipsis
7. Generate the actual pager
8. Add an ellipsis
9. Create the "next" and "last" links
10. Set theming ArrayPI values
11. Set cache context

Steps to reproduce

N/A

Proposed resolution

Provide a helper class to generate the pager variables, with logically split up methods and properties.

Remaining tasks

• Determine the best structure
• Implement

User interface changes

API changes

Data model changes

Release notes snippet